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No this is not normal child talk, and its better to nip this in the bud before he orders her to do something that could cause additional problems or hurt herself. It is possible that he has told her to not learn her words, though make sure you do not accuse anyone of this just yet.
What I would suggest is you first talk to your daughter, let her know that while she is to listen to the older children about her school work she should not follow orders that are not in relation to her schooling. (Such as sucking her thumb or doing anything that would hurt her or someone else).
I would then set up a appointment with her teacher and let her know what is going on. Explain the situation about the thumb sucking and mention the trouble you are having with her work as well. Let the teacher know that while you understand the need to enlist the older students you are concerned about what these students are ordering the younger students to do. Make sure that the teacher also speaks with your daughter and lets her know that it is ok to tell the older students No when they order her to do something that is not right.
These problems she has been having may very well be related to this situation. If she feels she is being forced to do these things she may be regressing which is not good at all. The sooner you get this nipped in the bud the better. I assume the teacher has no idea that the older child is using this against the younger children so you need to bring this to her attention.